Feedstocks containing higher molecular weight hydrocarbons are cracked by contacting the feedstocks under elevated temperatures with a cracking catalyst whereby light and middle distillates are produced. Typically, the yield ratio of light distillate (gasoline) to middle distillate is dependent upon the conversion level, therefore to increase the make of middle distillate, a corresponding decrease in conversion must be experienced. Unfortunately, this decrease in conversion requires significant changes in operating conditions which can have a detrimental impact on gasoline octane quality, or a change in catalyst type which can be time consuming and costly. Furthermore, presently available techniques for lowering the conversion level in a cracking operation, such as lowering reactor temperature, result in poor selectivity to the desired middle distillate product, and instead, lead to high yields of undesirable heavy, 650.degree. F.+ slurry oils. Therefore, with the current increase in demand for middle distillate fuels, it is desirable to have a modified cracking process available for quickly and reversibly changing from a maximum gasoline mode of operation to a maximum middle distillate mode of operation while minimizing loss in the octane number of the gasoline, to meet both seasonal and longer term fluctuations in the relative demand for distillate products
Present and forecasted future trends in the petroleum industry indicate significant changes in the demand patterns for petroleum products. The demand for gasoline has declined considerably, and is expected to decline further in the future. On the other hand, the demand for middle distillate products is on the rise. The fluid catalytic cracking process was invented to meet growing demand for gasoline in the 1930's and 1940's, and has traditionally been a process for maximizing the yield of gasoline from petroleum derived charge stocks. With the changes in demand trends described above, the present invention contemplates a new mode of operation of the fluid catalytic cracking process to advantageously shift yields to meet product demand changes.
It is thus one object of this invention to provide a regenerated cracking process, and a further object of this invention to provide a process for reversibly modifying the yield distribution of products from the process. Another object of this invention is to shift the yield distribution associated with a cracking process toward middle distillate.
Yet another object of this invention is to provide a process for switching from a maximum gasoline mode to a maximum distillate mode of operation, and back again to a maximum gasoline mode in a quick and reversible manner.